Single Photon Emission Computed Tomography imaging, also called “SPECT”, attempts to produce a three-dimensional reconstruction of the intensity of the three-dimensional distribution of a photon-emitting element within the body of the subject being imaged. Typically the emitted photons are gamma rays emitted from a radiopharmaceutical. SPECT systems generate such three-dimensional reconstructions by applying one of several known computerized reconstruction techniques to multiple projection images acquired around the subject. Exemplary techniques include filtered back-projection, ordered subset expectation maximization (OSEM) and other iterative techniques.
Photons are emitted into the region outside the subject being imaged during SPECT. These photons are referred to herein as “external photons”.
The mass of the subject attenuates the photons emitted within the subject (“internal photons”). Since the external photons are internal photons not lost to attenuation within the subject, the external photon intensity may vary from projection-to-projection.
If the attenuation properties of the mass along the path of each internal photon are known, a tomographic reconstruction technique can use this information to correct projection images for attenuation of the internal photons. This procedure is called attenuation correction.
Transmission computed tomography (“CT”) can be used to determine the attenuation properties along any internal photon's path within the subject. CT typically employs x-ray or gamma ray photons to produce essentially an attenuation map of the mass of the subject within the volume being imaged. A transmission attenuation correction map can be produced in this way.
Conventional large Field-of-View Anger style gamma cameras equipped with transmission attenuation correction often use the emission detectors as transmitted photon detectors. The photon source for transmission attention correction is often one or more isotopic gamma ray sources. The logic used to determine the location and energy of photons impinging on the external photon detectors in Anger cameras may limit the maximum count rate of the system, resulting in density maps with relatively poor precision. The FOV of these detectors may be as wide as 50 cm. This may enable collection of the transmission projection with little or no truncation. Truncation occurs when some portion of the volume being measured is not completely sampled in the projections.
Other approaches to transmission attention correction are known.